The present invention relates in general to alloy processing methods. It contemplates a process for preparing billet stock or other useful articles by the consolidation of particulate material and is particularly applicable to the preparation of sound, homogeneous components from the constitutionally complex alloys.
In the gas turbine engine industry the stringent strength and temperature demands made of the engine alloys, particularly in recent years, has placed this industry in the forefront of high temperature alloy development. Many of the advanced materials may be classified as constitutionally complex, i.e., they contain a large number of different elemental constituents all of which serve a definite advantageous purpose when properly combined and utilized. However, the highly alloyed nature of many of these materials has seriously complicated their production in usable form.
The production and evaluation of these alloys on a laboratory scale is often reproduced, if at all, only with difficulty on the production line. In the patent to Moore et al. U.S. Pat. No. 3,669,180, for example, it was pointed out that a lack of metallurgical homogeneity in actual wrought gas turbine engine hardware has led not only to a reduction in mechanical properties but also to a lack of predictability of such properties because of a broad scatter of sizes, types and locations of microstructural heterogeneity. This heterogeneity may in many cases be traced directly to a fault in the forging stock itself. For example, the slower solidification rate of a large ingot may provide sufficient time for undesirable localized constituent segregation which is not evident in smaller ingots wherein solidification has been more rapid.
In an effort to minimize the problems of heterogeneity and the resulting property scatter to permit a more reasonable utilization of the properties of which the idealized composition is capable of providing, resort has recently been made to a powder metallurgy approach to billet production. The rationale here is, of course, that if the individual powder particles are homogeneous, a billet formed of such powders should also be homogeneous. Unfortunately, such is not always the case, and the powder metallurgy approach has generated problems of its own.
Alloy powders may be generated by any one of several techniques. In general, however, two basic approaches are involved. In the first, very fine spheroids of the molten alloy are formed, as by expulsion of the melt through a nozzle, and quickly frozen, as illustrated by the technique of Gow in U.S. Pat. No. 2,439,772. In the second, a solidified ingot of the appropriate composition is comminuted to fine powder as suggested by, for example, Williams et al. U.S. Pat. No. 3,554,740 or Voightlander et al. U.S. Pat. No. 1,800,122.
Fundamentally, the powder metallurgy approach is inherently expensive and requires meticulous care in processing. Furthermore, even with the most careful attention to the processing details, problems of contamination and resistance of the powders to consolidation into a sound billet, as well as other problems associated with the inherent reactivity of fine powders, have often been evidenced in powder metallurgy products.
The overall result is that an urgent need still exists for a process that cannot only produce a satisfactory product from the constitutionally complex alloys but also one that will do so economically.